1. Field of the Invention
The present invention relates to a method for producing expanded polypropylene resin particles, wherein expandable polypropylene resin particles containing a physical blowing agent are released and expanded at reduced pressure from the inside of a pressure-tight vessel to give expanded particles. More particularly, the invention relates to a method for producing expanded polypropylene resin particles in which the particles are prevented from fusing together in the step for expanding the resin particles, and the resulting expanded particles fuse together better when the expanded particles are molded.
2. Description of the Related Art
Moldings comprising polypropylene resin formed into a desired shape (hereinafter referred to as expanded particle moldings) enjoy the advantages of properties such as impact resistance and heat insulation without sacrificing the outstanding properties inherent to polypropylene resin, and are thus widely used as packaging materials, construction materials, heat insulation materials, and the like.
Compared to unexpanded resin moldings, expanded particle moldings are more light-weight and are less expensive starting materials, and they are also endowed with properties such as impact resistance and heat insulation, making them even better in terms of cost performance. On the other hand, although expanded particles and molded articles are light-weight, they are bulky, resulting in the problem of high transportation costs. Particularly when expanded particle molding products manufactured in Japan are shipped for sale overseas, the extremely high transportation costs result in the loss of the exceptional cost performance inherent to expanded particle moldings. Thus, in order to minimize the costs associated with transport, expanded particle moldings for sale overseas are generally produced there, from the production of the expanded particles to the production of the expanded molding.
However, such overseas production, from the expanded particles to the expanded moldings, often results in the problem of considerable variation in the quality of the expanded particle molding products due to differences between production plants.
In a commonly used method for producing expanded polypropylene resin particles, resin particles dispersed in aqueous medium are impregnated with a blowing agent in a pressure-tight vessel such as an autoclave, and the resin particles impregnated with the blowing agent are then released and expanded in a reduced pressure atmosphere from inside the vessel at a temperature which is at least the softening temperature of the resin particles. The primary materials used in the production of expanded particles are the blowing agent, aqueous medium, and polypropylene resin particles, but a dispersant is added to the aqueous medium beforehand to prevent the softened resin particles from fusing together or from adhering to the inner wall of the vessel during the step for heating the resin particles in the vessel. When a dispersant is added to the aqueous medium, a portion of the dispersant that is used adheres to the surface of the resulting expanded particles, but if too much adheres to the surface of the expanded particles, the resulting expanded particles will not fuse together when molded. A resulting problem is the lower mechanical strength of the expanded particle moldings that are obtained by molding such expanded particles. Studies by the inventors have shown that the minimum amount of dispersant (the minimum amount needed to prevent the resin particles from fusing together during the production of the expanded particles) vary considerably from plant to plant overseas, where the difference between plants using the lowest and greatest amounts of dispersant can be as much as 5-fold. The use of different amounts of dispersant from plant to plant can result in different amounts of dispersant adhering to the expanded particles from plant to plant. Such inconsistencies between the amounts of dispersant adhering to expanded particles and the amounts of dispersant used may be the cause of differences in the quality of the expanded particle moldings that are obtained by molding the expanded particles.